1. Field of the Invention
The present invention relates to a waveplate such as a ¼ waveplate, a ½ waveplate and a full waveplate (optical waveplate) and to a waveplate manufacturing method.
2. Description of the Related Art
A waveplate used for a variety of optical apparatuses has hitherto been manufacturing by polishing a crystal of quartz. The waveplate is adjusted to such a thickness that a phase retardation between an ordinary light and an extraordinary light becomes an (N+¼) wavelength in the ¼ waveplate, an (N+½) wavelength in the ½ waveplate and an N wavelength (N is an integer) in the full waveplate.
Other than these types of waveplates, a waveplate utilizing form birefringence is proposed in (D. C. Flanders, “Submicrometer Periodicity Gratings as Artificial Anisotropic Dielectrics,” Applied Physics Letters, vol. 42, pp. 492-494, 15 Mar. 1983). The waveplate utilizing the form birefringence has gratings (micro-periodic structure) arranged with a smaller period than a wavelength (usage wavelength) of an incident light. The waveplate makes use of a phenomenon that let d be a grating pitch and λ be a usage wavelength, in an area where the grating pitch d is smaller than the wavelength λ, a refractive index np in a direction parallel with the grooves of the gratings is different from a refractive index nc in a direction orthogonal to the grooves of the gratings.
According to (D. C. Flanders, “Submicrometer Periodicity Gratings as Artificial Anisotropic Dielectrics,” Applied Physics Letters, vol. 42, pp. 492-494, 15 March 1983.) given above, when a sectional shape of the grating is rectangular, the refractive indexes np and nc are given by the following equations.np=[n12q+n22(1−q)]1/2  (1)nc=[(1/n1)2q+(1/n2)2(1−q)]1/2  (2)where n1 is a refractive index of a medium 1, n2 is a refractive index of a medium 2, q is an occupying ratio of the medium 1 during one period of the grating, and such a relation is established as 1≧q≧0.
A magnitude Δn of the birefringence is given by the following equation.Δn=|np−nc|  (3)
Further, a phase retardation ΔΦ received by the light having the wavelength λ, which gets incident on the gratings with the magnitude Δn of the birefringence, is given by the following equation, where D is a groove depth of the gratings.ΔΦ[rad]=(2πD/λ)·Δn  (4)From this equation (4), for obtaining a large phase retardation ΔΦ, the groove depth D of the gratings may be increased, or the magnitude Δn of the birefringence may be increased. This relation is established when the section of the gratings takes a sine-wave shape or a triangular-wave shape as well as when the section thereof takes a rectangular shape.
Mainly, the following two methods are given as methods of concretely manufacturing, based on the principle described above, the waveplate utilizing the form birefringence.
The first manufacturing method is that, at first, a photoresist is formed with the gratings by exposing the photoresist to the light on the basis of an interference exposure method, and a die is manufactured from the gratings. Next, the die is transferred onto a thermosetting resin or a photo-setting resin by a hot pressing method or an injection molding method, thus manufacturing the waveplate.
The second manufacturing method involves, in the same way as by the first manufacturing method, forming the gratings of the photoresist on a dielectric substrate. Next, however, with the photoresist being used as a mask, the dielectric substrate is etched by an ion etching method or a reactive ion etching method, thereby forming the gratings on the surface of the dielectric substrate. The waveplate is thus manufactured.
In the case of manufacturing this type of gratings by the first manufacturing method, a contact surface area between the medium and an electro-casting die conspicuously increases, and consequently a pull shearing force for exfoliation from the die surface rises. Therefore, the medium hardened when exfoliated is peeled off the dielectric substrate and remains on the die surface, resulting in a problem that the gratings can not be transferred with high accuracy.
Moreover, the second manufacturing method entails a long period of time for etching, and hence a thickness of a mask for a durable-against-etching photoresist increases, with the result that the mask for the photoresist is hard to form.
The gratings formed on the photoresist are transferred onto a substance, e.g., chromium, exhibiting high durability against etching, and, in the case of etching with this substance being used as a mask, the following problems arise.
Namely, if a groove of the grating is deepened, active species, ions and neutral particles, which reach the bottom of the groove, decrease in their numbers, and a progress of the etching is thereby hindered. Then, the gratings taking a desired sectional shape get hard to form.
This type of problem is caused without depending on the sectional shape of the gratings.
If the dielectric substrate is large in size, a problem is that intra-surface uniformity based on etching goes under an acceptable level, and a yield declines.
To cope with these problems, Japanese Patent Application Laid-Open Publication No. S62-269104 discusses a method of decreasing the depth of the groove by covering the gratings with a dielectric medium having a high refractive index. In this method, however, the groove has a narrow width, and it is therefore difficult to grow the dielectric medium down to the bottom of the groove.
Further, another known method is that the groove depth is decreased by manufacturing the gratings by using the dielectric medium having the high refractive index as a material. However, in the case of the dielectric medium having the high refractive index, the reflectance rises, and it is difficult to obtain the necessary and sufficient transmittance in terms of the optical element.
A known method contrived for solving this problem is a method of growing the dielectric medium having the small refractive index on the surface, however, reflection and scattering occur on an interface between the dielectric medium having the high refractive index and the dielectric medium having the low refractive index, and it is hard to acquire the transmittance that is as high as expected.